Cost of upgrading the anniversary coin is 32,400 without fails.
Here is a list of each level and its cost:
Upgrade cost to +1: 2000
Upgrade cost to +2: 2400
Upgrade cost to +3: 2800
Upgrade cost to +4: 3200
Upgrade cost to +5: 3600
Upgrade cost to +6: 4000
Upgrade cost to +7: 4400
Upgrade cost to +8: 4800
Upgrade cost to +9: 5200